It’s long been known that with a clear view we can see only 5% of space. By looking at the movements of galaxies,as well as gravitational lensing, we can infer that they have substantially more mass than can be seen by telescopes. It was postulated that this might be taken up by dead stars, but careful surveys in our own galaxy have shown that these make up only a very small proportion. 25% of the universe seems to be dark matter, something only detectable by its gravitational effects. Dark energy makes up the other 70%, and is a different beast altogether – whatever is causing the accelerating expansion of the universe seems to be smoothly distributed, and that’s pretty much all that’s known. Dark matter is at least something we can get a handle on: some kind of particle we haven’t detected yet, presumably.
But what if dark matter isn’t caused by another particle? What if current gravitational theory is wrong for large distances, making it look like there’s far more mass than really exists? This has always been a possibility. However, a new study has revealed positive evidence for the existence of dark matter, a major development for something which could only be inferred up until now. It’s still possible that gravitational theory is wrong, but this seems less likely with confirmation of dark matter’s existence.
Cosmologists looked at two clusters of galaxies which collided 100 million years ago – very recent in cosmological timescales. Most of the visible mass of galaxy clusters is made up of hydrogen between individual galaxies. Dark matter does not, it is thought, collide with normal matter, so it was theorized that in a collision between clusters the gases would slow each other down but leave dark matter untouched, as shown in this animation. Eventually the mass of the dark matter would re-attract the gas, but a recent collision would show a large amount of dark matter offset from the gas clouds. And so it proved. There’s definitely something there.
It’s very cool. Physicists and cosmologists at least know there’s really something to detect, and aren’t worried about a refinement of gravitational theory rendering years of searching pointless.
Cosmic Variance explain it all with much more detail.